The present invention relates to amplifiers for driving reactive loads, such as piezoelectric transducers.
Piezoelectric transducers transform electrical energy into mechanical energy and vice versa. Piezoelectric material stores electrical energy that is not converted into mechanical energy. This stored electrical energy is typically larger than the converted energy. The impedance of a piezoelectric transducer is predominantly capacitive, so that the energy is stored as charge on the capacitor. When a time-varying signal is applied from an amplifier, the capacitive load charges when the voltage rises and discharges when the voltage falls. During discharge, energy flows out of the load and into the amplifier.
In class A or B amplifiers, there is no provision for capturing and saving energy coming out of the load for later reuse when energy flows back into the load. Rather, energy flowing out of the load is converted to heat, resulting in inefficiency.
There are at least two amplifier design approaches that employ energy recovery and reuse—multi-rail amplifiers and switching (class D) amplifiers.
An example of a multi-rail amplifier used in such an application is disclosed in U.S. Pat. No. 5,264,752 issued Nov. 23, 1993 to Savicki. A multi-rail amplifier has many power supplies, providing a number of closely spaced power supply voltages, that can absorb energy and store it for reuse. A linear amplifier is disposed between each rail, with only one of these being in its linear operating range at any given time. The others are railed and consequently have low power dissipation. Net dissipation is reduced because the amplifier that is dissipating has a voltage drop that is only a fraction of the maximum output voltage swing. Efficiency increases with the number of rails. Energy is being reused because current inflow from the load is used to charge storage capacitors in the power supplies having rail voltages below that of the amplifier operating in its linear region. Energy storage is in electrolytic capacitors that are not deeply cycled. Thus, disadvantageously, a relatively large amount of physical space is taken up by the storage capacitors.
An advantage of multi-rail amplifiers is that they present much less severe design problems than do switching amplifiers. In particular, while switching does occur in the multi-rail amplifier, the ideal signal is intrinsically continuous, resulting in a much lower level of high frequency signal components and consequently reduced electromagnetic interference. Among the disadvantages of the multi-rail approach is that in order to achieve comparable efficiencies to switching amplifiers, 20 to 40 rails are required. The resulting high component count means that a multi-rail amplifier solution is useful where volume and cost are not major considerations. For that same reason, however, use of a multi-rail amplifier may be disadvantageous, or completely impractical, in applications having stringent volume requirements, such as submarine-based sonar systems, or where cost is a major concern.
Switching amplifiers, by contrast, are at least somewhat more compact and inexpensive. Switching amplifiers commonly operate by connecting a power supply voltage (Vsupply) across the load with alternating polarity. That is, the voltages Vsupply and −Vsupply are alternately impressed across the load. This switching is accomplished using, for example, power field-effect transistors (FETs) or insulated gate bipolar transistors (IGBTs) arranged in an H-bridge or full bridge configuration. The bridge is switched by a pulse-width-modulated (PWM) signal generated, for example, as a function of a high-frequency switching waveform and a baseband signal with which the load is to be driven. An amplified version of the baseband signal is thus impressed across the load by virtue of the fact that the average voltage across the load depends on the percentage of the time that the supply voltage is connected across the load with each polarity, this being a function of the baseband signal.
The signals appearing in a switching amplifier are within two distinct frequency bands—one at the frequencies of the load signal (baseband), and the other at the much higher switching frequencies (switching band) which start at and extend upward from the fundamental frequency of the switching waveform, which is also the fundamental frequency of the pulse-width-modulated signal. In a high performance amplifier suitable for baseband frequencies up to 100 kHz, the fundamental switching frequency will be at least 1 MHz, and the overall switching signal within the amplifier will have significant frequency components as high as 100 MHz.
In the optimal switching amplifier application, as much current or energy flowing out of the load as possible is routed back to the power supply. The power supply is designed to store the inflowing charge, thereby capturing energy for reuse. A conventional way to store energy in the power supply for reuse is through electrolytic storage capacitors. When current is flowing out of the load and into the power supply, the storage capacitor charge is slightly increased. This charge then flows out of the storage capacitor when energy needs to be sent to the load.
Although the switching amplifier solution does not involve the high component counts characteristic of multi-rail amplifiers, it also has its drawbacks.
In particular, the variation in current flow through the power supply resulting from the flow of baseband current to and from the load can cause variations in power supply voltage, giving rise to load signal distortion. Such “ripple” currents—not to be confused with the ripple voltage on a power supply's DC value that result when a power line frequency AC power signal is rectified—can be large, requiring the use of physically large storage capacitors in order to smooth them out. Indeed, the storage capacitors often dominate the volume of the power supply. Thus although the amplifier switching circuitry can be small, the physical volume of the electrolytic capacitors in the power supply can be many times the physical volume of the circuitry. As with the multi-rail approach, this is a disadvantage in applications in installations where physical space is at a premium.
A further drawback of the switching amplifier approach arises when the switching amplifier is used to drive capacitive loads. In particular, in any switching amplifier application, low pass load filters must be placed between the switching circuitry and the load in order to block high frequency currents from flowing through the load. Such currents will otherwise cause dissipative loss, damage the load and impair the function of the semiconductor switches. When the load is inductive, as is the case when the switching amplifier is used to drive a motor, for example, the load itself can provide some of the switching frequency filtering. Indeed, the load filter design principally needs to be concerned with providing effective filtering at those switching signal frequencies that are so high that the load no longer appears inductive, e.g., at higher order harmonics of the switching signal.
However, the impedance of a capacitive load, such as a piezoelectric transducer, decreases with frequency. In fact, in piezoelectric transducer applications, the load is essentially a dead short at the switching frequencies. A more complex and physically larger load filter is thus required for capacitive loads. The required high impedance of the load filter at the switching frequencies is typically supplied by one or more inductors. Since these inductors must carry the full baseband currents flowing to the load, they must be made physically large enough to prevent magnetic core saturation by the baseband currents. This constraint is, again, a problem in applications where physical space is at a premium.
Because the operation of a switching amplifier involves high frequency switching signals, e.g., signals in the 1-100 MHz range as described above, a switching amplifier is a radio-frequency (RF) device. Thus a further concern in the design of switching amplifiers in that the design of the switching amplifier must take into account such RF effects as electromagnetic interference and parasitic component complications. Designing the load filters is thus in large part an RF design problem and it can be difficult to achieve the desired load filtering functionality while addressing the aforementioned or other RF effects.